As a broadband optical access technology, the PON features a point-to-multipoint physical topology, which consists of an Optical Line Terminal (OLT), an Optical Distribution Network (ODN) and multiple Optical Network Units (ONUs). The multiple ONUs share optical resource and an OLT port. The ODN is connected passively with an OLT and one or more ONUs. An optical branch point in the ODN needs no active nodal device but a passive optical splitter. Consequently, the PON has such advantages as sharing bandwidth resource, saving investment of machine room, a high security of device, rapid networking and a low cost of comprehensive networking.
As the demands for broadband services increases, PON technologies are developing continuously, such as from the Asynchronous Transfer Mode (ATM)-based PON (ATM-PON, abbreviated as APON) to the Broadband Passive Optical Network (abbreviated as BPON) and further from the Ethernet-PON (abbreviated as EPON) to the Gigabit-capable Passive Optical Networks (G-PON, abbreviated as GPON) with the increase of transport bandwidth. The bandwidth of the existing GPON can be up to 2.5 Gbits/second (bps) for downstream and optional various rates of 2.5 Gbps, 1.5 Gbps, and 622 Mbps for upstream.
The GPON is a PON system initiated by the Full Service Access Network (FSAN) organization and established by the ITU-T Standardization organization. The GPON has the following features in terms of its functionality and performance: it can flexibly provide multiple symmetric or asymmetric upstream and downstream rates, such as 1.244 GBPS for upstream and 2.488 GBPS for downstream; a splitting rate of the system may be 1:16, 1:32, 1:64 and even 1:128, and the upstream and downstream rates are related with the Forward Error Correction (FEC) supported by the GPON; the GFP may be adaptable to any data service; it can well support transport for TDM service data, and provide a good guarantee for timing performance; it provides a perfect Operation, Administration, Maintenance and Provisioning (OAM&P) capability.
The GPON, as an access network, has numerous advantages; an appropriate transport system, however, shall be needed for cooperation with the GPON. An Optical Transport Network (OTN) is a highly reliable and interoperable high speed optical network, and can be taken as a backbone network or a metropolitan area network for cooperation with the GPON.
With respect to the OTN network, a client signal over the OTN is transported in the following three manners.
(1) Constant Bit Rate (abbreviated as CBR), i.e. CBR2.5G, CBR10G or CBR40G signals are mapped into an Optical channel Payload Unit (abbreviated as OPUk), in which CBR2.5G is a signal of a constant bit rate 2488320 kbit/s±20 ppm.
(2) Asynchronous Transfer Mode (abbreviated as ATM), i.e. ATM cells are multiplexed into fixed bit streams which match the payload capacity of an Optical channel Transport Unit (OPUk), and the bit streams are mapped into the OPUk. In multiplexing, the rate is adjusted through inserting idle cells or discarding cells. Information of the ATM cells should be scrambled prior to mapping.
(3) General Framing Procedure (abbreviated as GFP), i.e. in mapping a GFP frame, an idle frame is inserted during encapsulation to achieve a continuous bit stream which matches the OPUk, in which scrambling should also be performed. Some other signals may be mapped into the OPUk, such as a client signal, a test signal, a common client bit stream signal, etc.
Considering that the GPON and the OTN are different transport systems with different frame formats and overheads, and are applied in different scenarios, a networking way has been provided in the prior art. As illustrated in FIG. 1, which shows an architectural schematic diagram of GPON and OTN networking in the prior art, in a passive Optical Distribution Network (ODN), a user-side device (e.g. a computer terminal, a phone set, a television set) is connected with ONU 1, and is capable of transmitting and receiving a service signal.
In an upstream direction, when the user-side device transmits a service signal to the ONU1 through an Ethernet frame (e.g. a Media Access Control (MAC) frame), the ONU 1 can encapsulate the MAC frame into a GEM frame (a PON internal frame generated by using a GPON encapsulation method), and then the GEM frame is mapped into a payload area of an upstream optical burst packet, which is then added with a Physical Layer Overhead upstream (abbreviated as PLOu), a Physical Layer Sequence upstream (abbreviated as PLSu), a Physical Layer OAM upstream (abbreviated as PLOAMu) and a Dynamic Bandwidth Report upstream (abbreviated as DBRu), to compose an upstream burst timeslot stream for transport in an upstream line. The burst timeslot stream is a GPON Transmission Convergence (GTC), and is located in a Transmission Container (abbreviated as T-CONT). It shall be noted that the GPON is a specific example of the PON. With respect to the GPON, the burst timeslot stream transmitted out from the ONU1 is signals in a GTC format. With respect to the general PON, the burst timeslot stream transmitted out from the ONU1 is signals in a PON frame format.
An OLT 2 is connected directly with the ONU1. When receiving the upstream burst timeslot stream, the OLT 2 extracts the PLOu, then extracts the GEM frame from the payload area, and removes the GEM encapsulation, thus recovering the original service signals in the MAC frame format. When a GFP adaptation protocol is adopted in an OTN 3, the OLT 2 has to firstly encapsulate the original service signals through the GFP, and then transmits the encapsulated service signals to an optical transport device 4 and further to another optical transport device 5 in the OTN 3. The optical transport device 5 transmits the service signals to a network serving party, i.e. a digital video network, the Internet, or a Public Switched Telephone Network (abbreviated as PSTN).
The processing in a downstream direction is similar to that in the upstream direction, and therefore is not described again. The inventors have recognized when making the present invention that for the transport procedure of service signals provided in the prior art, the GEM is just an internal adaptation protocol of the GPON, and is generated and terminated only between the ONU and OLT, while the GFP is just an internal adaptation protocol of the OTN network, and is greatly different from the GEM in terms of their formats and functionalities, thus an integration of network elements is difficult, and even with a physical integration, they may be logically independent from each other, which will be adverse to a mutual integration of a transport network with an access network.
Further, with respect to the networking way, the service signals encapsulated into the GEM frame can be of an access to the OTN only after they are recovered into the original service signals through the OLT, and the GPON is disadvantageous in a short transport distance and support for a limited number of users, a large number of OLTs have to be configured separately in sites which are located very dispersedly, thereby resulting in a very high cost of network operation and maintenance.